a. Field of the Invention
The present invention relates generally to installation of underground pipelines and similar conduits, and, more particularly, to an apparatus for installing fill material around a pipeline after this has been placed in an excavated trench.
b. Related Art
Underground pipelines are used for many purposes, including oil and natural gas lines, water lines, sewer lines and electrical power conduits, for example. In most instances, such pipelines are installed by excavating a trench in the earth, laying the pipe in the trench, and then placing protective fill material around and over the pipe.
The protective fill material is normally one of two types. For oil and gas lines and similar types of high-strength pipe, the fill material is ordinarily a soil or gravel material which is screened for size, but which does not necessarily include a cement or other binder component. The padding in such installations serves mostly to protect the pipe from coming into contact with large/sharp rocks in the subterranean formation or in overlying layers of backfill. The padding also protects the pipe from vertical loads, such as the weight of the overlying fill or that of a vehicle crossing over the trench, by directing these loads outwardly into the earth along the sides of the trench. Still further, the padding serves to protect the pipe against excessive pressures in the event of a shifting or collapse of the earth along the trench, as due to an earthquake for example, and also protects the cathodic protection coating on the pipeline material itself.
In order to serve these functions, the fill material must be screened to exclude pieces of rock greater than a predetermined size, and the material must then be distributed carefully in a predetermined profile over and around the pipe. Also, depending on the specifications for the installation, different (e.g., coarser or finer) grades of material may need to be installed in layers to form the fill, and cement and/or fly ash may also be included in one or more of the layers to provide a stronger, more coherent padding. Furthermore, in some installations it is necessary to add a certain amount of water to the padding material so as to form a slurry which is able to flow in and fill completely around and underneath the pipeline (e.g., see FIG. 3).
To obtain the fill material, excavated earth (often, the native spoils excavated from the trench itself) is trucked or otherwise transported to a screening plant. After screening, the material is transported back to the trench, where it is dumped onto the pipe using a truck or loader. Not only is such a process time consuming and inefficient, but simply dumping the material onto the pipe in this manner makes it very difficult to provide the fill with the proper contour.
Certain machines have been developed for the specific purpose of screening and installing pipeline padding, but these have generally been unsatisfactory in one or more respects. To illustrate these problems, a typical prior art pipeline padder 10 is shown in FIG. 1; machines of this type are available from several sources, including Ozzie's Pipeline Padder Co., 1545 West Watkins, Phoenix, Ariz.
As can be seen in FIG. 1, prior art padders typically include some form of tracked carriage 12 which propels the vehicle alongside the trench 14 (in some instances there is a separate bulldozer or other tracked vehicle which carries or propels the assembly), in the direction indicated by arrow 16. As the machine moves along, an on-board conveyor 18 picks up excavated backfill lying alongside the trench and discharges this on top of a vibrating screen shaker 22. Fines in the backfill pass through the screen shaker, while larger rocks and debris roll off the screen and are discarded as indicated at 24.
The screened material falls onto a transverse belt 26 which transports the material sideways towards the trench at a comparatively high speed. A deflector plate 28 mounted at the end of the conveyor assembly intercepts the material and redirects this downwardly on top of the pipeline 30 so as to form the padding 32.
While machines of this type are in widespread use, their efficiency is limited by a number of problems which are inherent in the basic configuration. One of the most serious drawbacks is the inability of the operator to effectively control placement of the material in trench. As can be seen, the operator 34 is located well off to the side of the trench, in a position where he is viewing the pipe at an angle and where his sight is partially obstructed by the upper edge of the trench. As a result, it is difficult or impossible for the operator to see whether the fill is being placed correctly at any given point. Moreover, the operator has only limited control over the position of the conveyor assembly, and the transverse alignment of the conveyor (shooting crosswise onto the trench) makes it very difficult to direct the flow of material with any degree of accuracy.
These deficiencies have become more acute with the development of more sophisticated pipeline padding or bedding techniques, some of which require the installation of several layers of fill material, each of which has its own specified depth and profile. For example, FIG. 2 shows an installation in which the first layer of fill 36 is formed of a comparatively fine material which surrounds the pipe and has a domed profile 38 along its upper surface, while the subsequent layers 40 and 42 vary in thickness and coarseness of the material. It will be appreciated that this form of padding requires precise, controllable placement of the different grades of fill material, which has been very difficult to accomplish using existing types of machines.
Furthermore, certain installations call for mixing water with the fill material, so as to make it more flowable, or for mixing in Portland cement or other cementitious material to form a stronger, more coherent fill. In particular, most water lines require the use of a cementitious fill material, commonly referred to as Controlled Low Strength Material (CLSM). Water lines are typically much thinner walled and weaker than gas or oil lines, and the shape and strength of the fill material, forms a significant part of the design strength of the pipe.
Existing types of pipeline padding machines, such as that described above, have no provision for adding and mixing water and cement with the fill so as to form a CLSM material. As a result, CLSM fill has typically been made at a local concrete ready-mix plant and then trucked to the installation site, where it is placed using the discharge chute of the mixer truck. This practice is extremely inefficient in several respects. Firstly, if the native spoils which have been excavated from the trench are to be used in the CLSM fill, these must be trucked from the excavation site to the to the ready-mix plant; otherwise, the excavated material must be disposed of and new aggregate material purchased to form the CLSM.
Furthermore, the CLSM begins to set immediately after mixing and must be deposited in the trench within a comparatively short time. Not infrequently, however, the bedding operation is interrupted for one reason or another (e.g., to make a repair or correction), or the operation has advanced to a section of the pipeline where the fill material is not needed. When this happens, the CLSM material from the ready-mix plant must be disposed of in short order, before it sets up inside the mixer truck. Not only does this cause the material to be wasted, but it is also necessary to find (or excavate) a hole or other cavity into which the material can be dumped.
Accordingly, there exists a need for a pipeline padding/bedding machine which enables the operator to accurately place the fill material in the desired positions and contour around and over a pipeline. Furthermore, there exists a need for such an apparatus in which water can be added to the fill material in an efficient, continuous manner, when this is needed in order to form a slurry or flowable padding which will flow completely under and around a pipeline and/or pipeline appurtenance. Still further, there exists a need for such an apparatus in which Portland cement or other cementitious materials can be incorporated in the fill material in an efficient, manner when these are needed, and on a continuous or interrupted basis as necessary.